Covered stent with encapsulated ends

ABSTRACT

A portion of a covered stent is encapsulated with ePTFE, so that the unencapsulated portion, which is covered by a single ePTFE covering, imparts an unimpaired flexibility to the stent. One surface of the stent, either the luminal or abluminal surface, is covered by a single continuous layer of ePTFE, while limited regions, preferably near the ends of the stent, of the other surface are also covered by ePTFE. The regions covered by ePTFE on both surfaces become encapsulated when the ePTFE of one layer becomes bonded to second layer. By leaving a middle region of the stent unencapsulated, the stent retains flexibility similar to a bare stent, thereby reducing the loading and deployment forces.

This application claims the benefit of U.S. Provisional Application No.60/118,269, filed Feb. 02, 1999, and is a continuation-in-part ofapplication Ser. No. 08/401,871, filed Mar. 10, 1995, now U.S. Pat. No.6,124,523, each of which is expressly incorporated by reference as iffully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the field of medical devices,and more particularly, to the encapsulation of stents.

2. Description of Related Art

Stents and similar endoluminal devices are currently used by medicalpractitioners to treat tubular body vessels or ducts that become sonarrowed (stenosed) that flow of blood or other biological fluids isrestricted. Such narrowing (stenosis) occurs, for example, as a resultof the disease process known as arteriosclerosis. While stents are mostoften used to “prop open” blood vessels, they can also be used toreinforce collapsed or narrowed tubular structures in the respiratorysystem, the reproductive system, bile or liver ducts or any othertubular body structure. However, stents are generally mesh-like so thatendothelial and other tissues can grow through the openings resulting inrestenosis of the vessel.

Polytetrafluoroethylene (PTFE) has proven unusually advantageous as amaterial from which to fabricate blood vessel grafts or prostheses,tubular structures that can be used to replace damaged or diseasedvessels. This is partially because PTFE is extremely biocompatiblecausing little or no immunogenic reaction when placed within the humanbody. This is also because in its preferred form, expanded PTFE (ePTFE),the material is light and porous and is readily colonized by livingcells so that it becomes a permanent part of the body. The process ofmaking ePTFE of vascular graft grade is well known to one of ordinaryskill in the art. Suffice it to say that the critical step in thisprocess is the expansion of PTFE into ePTFE. This expansion represents acontrolled longitudinal stretching in which the PTFE is stretched toseveral hundred percent of its original length.

Apart from use of stents within the circulatory system, stents haveproven to be useful in dealing with various types of liver disease inwhich the main bile duct becomes scarred or otherwise blocked byneoplastic growths, etc. Such blockage prevents or retards flow of bileinto the intestine and can result in serious liver damage. Because theliver is responsible for removing toxins from the blood stream, is theprimary site for the breakdown of circulating blood cells and is alsothe source of vital blood clotting factors, blockage of the bile ductcan lead to fatal complications. A popular type of stent for use in thebiliary duct has been one formed from a shape memory alloy (e.g.,nitinol) partially because such stents can be reduced to a very lowprofile and remain flexible for insertion through the sharp bend of thebile duct while being, self-expandable and capable of exerting aconstant radial force to the duct wall.

Cellular infiltration through stents can be prevented by enclosing thestents with ePTFE. Early attempts to produce a stent covered by ePTFEfocused around use of adhesives or physical attachment such as suturing.However, such methods are far from ideal and suturing, in particular, isvery labor intensive. More recently methods have been developed forencapsulating a stent between two tubular ePTFE members whereby theePTFE of one-member touches and bonds with the ePTFE of the other memberthrough the mesh opening in the stent. However, such a mnonolithicallyencapsulated stent may tend to be rather inflexible. Therefore, there isa need for a stent covered to prevent cellular infiltration and yetstill flexible to ensure ease of insertion and deployment and toaccommodate extreme anatomical curves.

SUMMARY OF THE INVENTION

The present invention is directed to covered stents wherein flexibilityof the stent is retained, despite the use of encapsulation techniques.Encapsulation refers to the lamination of a stent between an inner andan outer layer of a plastic material. Compared to a fully encapsulatedstent enhanced flexibility can be achieved by encapsulating limitedregions of the stent, while leaving a significant portion of thestent—usually a middle portion—covered by a single layer of the plasticmaterial. In this way the limited encapsulation fixes the plasticcovering onto the stent with no need for sutures or similar laborintensive mechanical attachments.

It is an object of this invention to provide a stent device that hasimproved flexibility compared to a fully encapsulated stent, yetmaintains its impermeability to infiltrating tissues.

It is yet another object of this invention to provide a stent devicethat shows minimal profile when loaded into insertion systems and can bedeployed using forces that are reduced compared to those used with fullyencapsulated designs.

These and additional objects are accomplished by embedding orencapsulating only portions of the stent between two layers ofbiocompatible material. This is accomplished by covering either theluminal or abluminal surface of the stent with a layer of biocompatiblematerial, preferably ePTFE, while also covering limited sections of theopposite surface of the stent with the biocompatible material, therebyfully encapsulating only the limited sections. A preferred design fullyencapsulates only the end regions of the device. By leaving a middleregion of the stent unencapsulated, the stent is free to flex much likea bare stent, increasing overall flexibility and reducing the necessaryloading and deployment forces.

In the present invention, a stent is partially encapsulated using theconfiguration mentioned above. One means of accomplishing thisconfiguration is to place rings (radial strips) of ePTFE on a mandrel atpositions corresponding to each end of the stent. The stent is thenplaced over the mandrel and the rings in registration with the ends ofthe stent. Finally, the stent (supported by the mandrel) is covered onits abluminal (outside) surface by a tubular ePTFE graft. The resultingstructure is then subjected to heat and pressure so that the regionscontaining ePTFE on both surfaces become laminated or fused together(e.g., a bond is formed). This yields a stent with substantially itsentire abluminal surface covered by ePTFE. Regions near the ends of thestent are fully encapsulated (e.g., these regions are covered by ePTFEon their luminal surfaces as well). The fully encapsulated area servesto attach the abluminal covering to the stent.

A more complete understanding of the partial encapsulation of stentswill be afforded to those skilled in the art, as well as a realizationof additional advantages and objects thereof, by a consideration of thefollowing detailed description of the preferred embodiment. Referencewill be made to the appended sheets of drawings, which will first bedescribed briefly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the preferred embodiment of the presentinvention.

FIG. 2 is a cross-sectional view along the line 2—2.

FIG. 3 is a cross-sectional view along the line 3—3.

FIG. 4 is an overview picture of the deployment of the device of thepresent invention.

FIG. 5 is a close-up view of the device being partially deployed.

FIG. 6 is a close-up view of the device fully deployed.

FIG. 7 is a picture of a fully encapsulated stent being tested forflexibility.

FIG. 8 is a picture of the covered stent of the present invention beingtested for flexibility in the same manner as FIG. 7.

FIG. 9 shows an especially flexible stent design (the “Flexx” stent)preferred for use in the present invention; here the Flexx stent isshown in its expanded state.

FIG. 10 shows the flexible stent of FIG. 9 after it has been compressed.

FIG. 11 shows a close-up of the strut structure of the expanded stent ofFIG. 9.

FIG. 12 shows a close-up view of the flexible stent design of FIG. 9immediately after being cut from a metal tube and before being expandedinto the form of FIG. 11.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention satisfies the need for a covered stent device thatis virtually as flexible as an uncovered stent. This is accomplished bycovering a stent on a first surface while limited regions are covered onthe opposite surface to ensure fixation of the first surface covering.

Referring now to the drawings, in which like reference numbers representsimilar or identical structures throughout, FIG. 1 illustrates apreferred embodiment of the present invention. A partially encapsulatedstent-graft 10 is created by covering the abluminal surface of a stent12 with a biocompatible barrier material that is able to seal fistulaeand aneurysms and prevent or reduce tissue ingrowth from neointimalhyperplasia or tumor growth. In the preferred embodiment, the materialused for this purpose is a tubular layer of expandedpolytetrafluoroethylene (ePTFE) 20. The preferred ePTFE is one optimizedfor bond strength as described in U.S. Pat. No. 5,749,880. The stent 12in the preferred embodiment is a shape memory alloy stent havinggeometry enhancing the stent's flexibility, although stents of a varietyof designs are usable with the current invention because the inventiveconfiguration minimizes the effect of the covering on stent flexibility.Also, the stent 12 can be made out of any type of material besides shapememory alloy.

It will be apparent to those of skill in the art that at a covering overat least one of the surfaces (luminal or abluminal) of the stent isnecessary to prevent tissue ingrowth. Furthermore, the covering must bebonded to the stent to prevent it from coming detached and perhapsforming a blockage in the vessel. Although ePTFE has numerous favorableproperties, it is relatively difficult to attach it to a stent.Mechanical fasteners such as sutures have the disadvantage ofinterrupting the integrity of the ePTFE sheet so that leaking can occur.Although ePTFE does not adhere well to a stent, it can be made to bondto itself. Therefore, one effective method of affixing the ePTFE coveris to place ePTFE covers in contact with both the abluminal and luminalsurfaces of the stent so that one ePTFE covering can bond to the otherwhere the ePTFE coverings touch through the openings in the stent. Thedrawback with this approach is that the structural members of the stentare tightly surrounded and held by ePTFE. When the stent bends orexpands, the stent structural members must move relative to each other.This movement is resisted by the tightly adhering ePTFE (or othercovering material).

In the present invention movement of the stent members relative to eachother is facilitated by limiting the region of the stent in which thestructural members are surrounded (encapsulated) by ePTFE. In apreferred embodiment the regions of encapsulation, which ensureattachment of the covering to the stent, are limited to areas near theends of the device. For a relatively short device these end-encapsulatedregions are more than adequate to afford attachment of the covering. Ifnecessary one or more additional regions of encapsulation could be addedalong the length of the device if it is found necessary for stability ofthe covering. Clearly, the greater the percentage of length of thedevice that is fully encapsulated, the more the flexibility of theoverall structure will be impeded.

An additional advantage of the limited encapsulation of the presentinvention is the possibility of enhanced healing. It is known thatliving cells will infiltrate sufficiently porous ePTFE and thatmicrocapillaries may form within and across the ePTFE wall so that aliving intima is formed along the luminal surface. Where two layers ofePTFE surround the stent it may be significantly more difficult forcellular infiltration across the wall to occur. Although the figuresshow the continuous covering placed on the abluminal surface of thedevice, the present invention also lends itself to placement of thecontinuous covering on the luminal surface. The configuration choice maydepend on the precise application of the device. In some applications,for example large vessels having a high rate of blood flow placing thecovering on the luminal surface may result in advantageous lamellar flowof the blood—blood flow without significant turbulence. There is someevidence that contact of the blood with a metal stent may result inlocal, limited thrombosis. While this may be detrimental, there is alsosome evidence that such limited thrombosis results in enhanced healing.An advantage of using a full luminal covering could be improvedanchoring of the device within the duct or vessel afforded byinteractions between the bare abluminal stent and the duct or vesselwall. Therefore, the optimal configuration will have to be empiricallydetermined in many cases.

In the illustrated design (FIG. 1) the extremities 14 of the stent 12are left completely uncovered and flare outward to facilitate anchoringof the stent within the vessel following expansion of the stent in situ.It will be apparent that this flared region is a feature of thisparticular embodiment and is not a required element of the instantinvention. The luminal surface of the stent 12 is covered at ends 22defined between points A and B and points C and D in FIG. 1, but is leftuncovered in mid-section 24 defined between points B and C. By leavingthe mid-section 24 uncovered, the stent has increased flexibility aswell as reduced profile when compressed. The material used to cover theends 22 on the luminal surface of stent 12 is generally the samematerial that is used to cover the abluminal surface, and in FIG. 1 thismaterial is ePTFE 30 (see FIG. 2), though any other suitablebiocompatible material could be used in the present invention.

Again, it is important to note that while the continuous tubular layerof ePTFE 20 is shown on the abluminal surface of FIG. 1, it is possible,and advantageous in some cases, to place a tubular layer of ePTFE on theluminal surface, while placing limited rings of ePTFE only on theabluminal surfaces at the ends of the device. Distances A-B and C-D inFIG. 1 can be lesser or greater, depending on the need for flexibilityin the particular application. Moreover, there can be any number ofencapsulated region(s) and these region(s) can be located in differentareas of the stent. Also, while the preferred embodiments useencapsulated regions that extend completely around a circumference ofthe device (e.g., rings of material) as indicated by region 32 in FIG.1, there is no reason that discontinuous regions of encapsulation cannotbe used. Attaching discrete pieces or strips of ePTFE to a mandrelbefore the stent is placed on the mandrel can be used to form suchdiscontinuous regions. The size, shape and pattern formed by regions 32can be selected to enhance flexibility, etc. This allows differentregions of the device to exhibit different properties of flexibility,etc.

Once the appropriate ePTFE covering is placed onto the luminal andabluminal surfaces, the ends 22 of the stent graft 10 are encapsulatedby connecting or bonding the luminal covering to the abluminal covering.Encapsulation can be accomplished by a number of methods includingsintering (i.e., heating), suturing, ultrasonically welding, staplingand adhesive bonding. In the preferred embodiment, the stent-graft 10 issubjected to heat and pressure to laminate (bond) the tubular ePTFElayer 20 on the abluminal surface to the two rings of ePTFE 30 on theluminal surface.

FIGS. 2 and 3 illustrate cross-sections of FIG. 1. A cross-section ofstent-graft 10 is taken along line 2—2, through an end 22 of the device10 in FIG. 2 and along line 3—3, through the mid-section 24 in FIG. 3.These two cross-sections are shown to illustrate the additional layer ofePTFE 30 that is present on the luminal surface of the end 22 and notpresent on the luminal surface of the mid-section 24. As mentioned, thereason for encapsulating only the ends 22 of stent-graft 10 is toincrease its flexibility over a fully encapsulated stent, therebyallowing it to be bent into extreme curves without kinking. Most of thelength of the device is covered by only a single layer of ePTFE which isextremely flexible and which does not strongly interact with the stent.Therefore, the flexibility of the single layer area is essentially thatof the underlying stent device. FIG. 7 shows a fully encapsulated shapememory alloy stent bent in essentially as sharp a curve as possible.Note that the covering material is showing kinks or distortions 34 dueto the inability of the covering material to move longitudinallyrelative to the stent structural members. FIG. 8 shows an identicalshape memory alloy stent covered according to the current invention:only the extreme device ends are fully encapsulated. Note that thedevice is capable of being bent into a much sharper curve with little orno distortion of the covering or the underlying stent.

An additional advantage provided by the present invention is that theretraction force necessary to deploy the stent-graft 10 using a coaxialdeployment system is drastically reduced in comparison to a fullyencapsulated stent. This is due to the reduction in amount of coveringmaterial. Furthermore, by reducing the amount of covering material, theoverall profile of the deployment system is reduced, allowing a widerrange of applications. Another advantage enjoyed by the presentinvention is its ease of manufacture compared to stent-graft devicesthat place multiple stent rings over ePTFE tubing. Finally, an advantageover stent-grafts with a single layer of biocompatible material over theentire graft length is that because a strong bond is created in theencapsulated region, it is possible to transmit a pulling force from oneend of the stent of the present invention to the other via the covering,making it possible to load into a sheath using pulling techniques. Thepreferred bare stent designs (chosen for flexibility and low profile) donot permit transmission of a pulling force in a longitudinal axialdirection. This is because flexibility is increased and profile reducedby removing connections between longitudinally neighboring struts. Thelimited number of longitudinal connections has inadequate tensilestrength to transmit the pulling force without failure. In the case of atrue single layer covering (without use of adhesive, etc.) pulling onthe covering causes the covering to slip off the stent. In the case ofsutured single layer device pulling on the covering may cause thesutures holes to enlarge and even tear.

EXAMPLE 1

Two memotherm (shape memory alloy stent, product of Angiomed, Divisionof C. R. Bard, Inc.) biliary stents (S1 and S2), partially encapsulatedaccording to the present invention, were loaded into a 10 Frenchdelivery system used for a standard covered biliary stent. The stentswere 10 mm×60 mm. The pulling force necessary to load the stents (theforce between the outer sheath and the stent) was measured as follows:

S1=6.3 N

S2=3.5 N

In comparison, the loading force for a fully encapsulated stent isapproximately 50 N. After loading the samples S1 and S2 into a pullbackdelivery system, both were deployed into a glass biliary duct modelplaced in a 37° C. water bath. All deployment went smoothly and nosignificant covering damage was observed. Thus, the partiallyencapsulated stents could be loaded employing a much-reduced forcewithout being compromised structurally.

EXAMPLE 2

Three prototypes (P1, P2, and P3) were built using a Gamma 2 (Flexx)design memotherm stent, 12 mm×120 mm. These prototypes were partiallyencapsulated according to the present invention. More particularly, theabluminal surface of each stent was covered with a tubular ePTFEmaterial, leaving the regions near the stent ends uncovered (to flareoutward and anchor the device). The luminal surface near each end of thestent was covered by a 9.95 mm±0.05 mm ring of ePTFE material. Thestents were then subjected to heat and pressure so that the overlappingePTFE material on the luminal and abluminal surfaces was bondedtogether. The prototypes were then loaded into a 10 French deliverysystem and were deployed into a glass biliary duct model (45°, 25.4 mmradius) that was placed in a 37° C. water bath.

The prototypes were loaded according to the standard loading techniqueused for loading fully encapsulated stents. This loading techniqueconsists of compressing the stents by, pulling them through a funnelusing specially designed hooks. When loading the fully encapsulatedstent, a backing mandrel and core are used to create a uniform foldedpattern in the compressed stent. In loading P1, no backing mandrel andcore inside the stent were used, resulting in an unacceptable load dueto the presence of folds. P2 was loaded using a backing mandrel (9.2 mmdiameter) and a core (1.25 mm diameter), resulting in a successful loadwith no folds. P3 was loaded in the same manner as P2. Loading forcesbetween the funnel and the stent and pulling forces between the stentand the outer sheath were measured as follows:

Prototype Peak Loading Force (N) Peak Pulling Force (N) P1 12.5 — P227.5 12.9 P3 18.5 14.8

The loading force and pulling force necessary to load and deploy theprototypes were much smaller than that necessary for a fullyencapsulated stent. Thus making it possible to load and deploy theprototypes with either a manual pullback or a pistol handgrip deploymentsystem.

In the case of a biliary stent an especially tortuous delivery path mustbe used. There are two main techniques for such delivery. If the stentis deliver transhepatically, it is inserted through percutaneousvasculature, through the bulk of the liver and down the hepatic ductwhere it must make a bend of around 45 degrees between the hepatic andthe bile duct. If the stent is delivered endoscopically it enters thebile duct via the papilla and must pass through multiple bends, the mostsever of which is about 90 degrees with a 10 mm radius. Clearly anextremely flexible stent is required. To further illustrate thedeployment of the prototypes, FIGS. 4-6 have been provided. FIG. 4 showsan overview of the prototypes being deployed into a glass model of abile duct using a pistol handgrip delivery system. Note the bend thatthe stent must navigate. FIG. 5 shows a close-up view of a prototype, asit is partially deployed from the sheath. FIG. 6 shows a close-up viewof a fully deployed prototype.

The “Flexx” stent used in these experiments is a specially designedstent configured for the present invention. Stents of this type are cutfrom tubes of Nitinol shape memory alloy and then expanded on a mandrel.The size memory of the device is, set on the expanded form. The deviceis then compressed to the approximate dimensions of the original tubefor insertion into a patient. Once properly located in the patient, thedevice is released and can self-expand to the “memorized” expandeddimension. Although the entire device is a single unitary piece, asshown in FIG. 9 in its expanded state, this design conceptuallycomprises a plurality of zigzag ring stents 64 (stenting zones) joinedby longitudinal joining points 62.

FIG. 10 shows the recompressed device to illustrate that each ring stent64 is attached to each adjacent ring stent 64 by only a pair of joiningpoints 64. Note the open regions 60 between the joining points 62. Itwill be apparent that such a structure affords considerable lateralflexibility to the entire compressed structure. If there were a largernumber of joining points 64, lateral flexibility of the compresseddevice would be impeded. On the other hand, the very open structure ofthe expanded stent (FIG. 9) offers little resistance to tissueinfiltration.

These two factors account for the unusual suitability of the Flexxdesign in the present invention. The use of a covering of ePTFE or otherbiocompatible material prevents tissue infiltration despite the veryopen nature of the Flexx design. The use of end encapsulation (asopposed to encapsulation over the entire length of the device) preservesmost of the inherent flexibility of the design. The use of only a singlelayer of covering over much of the stent results in a low profile in thecompressed configuration so that the device can be inserted throughsmall bile ducts and other restricted vessels. The use of only a verylimited number of joining points 64 provides the lateral flexibilityrequired for insertion through tortuous bile ducts and other similarlytwisted vessels.

FIG. 11 is a close-up of a portion of FIG. 9 and shows the adjacent ringstents 64 (stenting zones) and the joining points 62. Each ring stent 64(stenting zone) is formed from a zigzag pattern of struts 54. Thesestruts have the thickness of the Nitinol tube from which the device islaser cut with a width, in this embodiment, of about 0.2 mm.

There is a joining point 62 between a given ring stent 64 and anadjacent ring stent 64 every third strut 54 with the joining points 62alternating from the left-hand adjacent to the right hand adjacent ringstent 64 so that six struts 54 separate the joining points 64 betweenany two ring stents 64. Gaps 32 replace the joining points 62 where theintersections of zigzag struts are not joined.

FIG. 12 shows a close-up of the non-expanded cut structure of FIG. 10.Cuts 40, 41, and 42 are regions where the metal has been vaporized by acomputer controlled cutting laser. The cut 40 between blind cuts 41 willexpand to form the window 60. Cut 42 forms the intersection point * ofthe struts 54, which show portions of two ring stents 64. Partially cutregions 55 define a scrap piece of metal 32′, which is removed followingexpansion to form the gaps 32. In the figure the partially shown regionabove the cut 40 and above the scrap piece 32′ is the joining point 62.Because a structure with only two joining pieces 62 between adjacentstent rings 64 is too fragile to withstand expansion as from FIG. 12 toFIG. 11, the scrap pieces 32′ act as reinforcing joining points for theradial expansion process. Following expansion the scrap pieces 32′ areremoved to form the gaps 32. This structure can then be deformed intothe reduced diameter flexible structure shown in FIG. 10. It will beapparent that although this structure is described and pictured ashaving circumferential ring stents 64, the stents zones can also bearranged in a helical manner to achieve the objects of the improveddesign.

Having thus described a preferred embodiment of the covered stent withencapsulated ends, it will be apparent by those skilled in the art howcertain advantages of the present invention have been achieved. Itshould also be appreciated that various modifications, adaptations, andalternative embodiments thereof may be made. For example, while Flexxstent designs partially covered with ePTFE have been illustrated, itshould be apparent that the inventive concepts described herein would beequally applicable to other types of stent designs and biocompatiblecovering materials. Moreover, the words used in this specification todescribe the invention and its various embodiments are to be understoodnot only in the sense of their commonly defined meanings, but to includeby special definition in this specification structure, material or actsbeyond the scope of the commonly defined meanings. The definitions ofthe words or elements of the following claims are, therefore, defined inthis specification to include not only the combination of elements whichare literally set forth, but all equivalent structure, material or actsfor performing substantially the same function in substantially the sameway to obtain substantially the same result. The described embodimentsare to be considered illustrative rather than restrictive. The inventionis further defined by the following claims.

We claim:
 1. An implantable prosthetic device, comprising: a stenthaving a luminal and an abluminal surface and a first and a second end;a first tubular layer of expanded polytetrafluoroethylene covering theabluminal surface of the stent, wherein said first tubular layer doesnot cover a region immediately adjacent the first or the second end ofthe stent; a second tubular layer of expanded polytetrafluoroethylene incontact with the luminal surface of the stent, wherein said secondtubular layer does not cover a majority of the luminal surface of thestent, said second tubular layer comprising at least one ring disposednear the first or the second end of the stent; and an encapsulatedregion wherein said first tubular layer bonds to said second tubularlayer.
 2. The implantable prosthetic device of claim 1, wherein saidstent comprises shape memory alloy.
 3. The implantable prosthetic deviceof claim 2, wherein said stent comprises zigzag struts forming stentingzones, wherein a plurality of scrap portions that join adjacent stentingzones are removed to form spacings between the adjacent stenting zones.4. The implantable prosthetic device of claim 1, wherein said secondtubular layer further comprises a second ring disposed at an oppositeend of the stent from said at least one ring, where a secondencapsulated region is formed.
 5. A process for producing a flexiblecovered stent comprising the steps of: providing a stent having a stentlength and luminal and abluminal surfaces; placing a first tubularcovering of expanded polytetrafluoroethylene, having a first coveringlength, in contact with the abluminal surface of the stent so that amajority of the surface is covered thereby, but wherein a regionimmediately adjacent the first or the second end of the stent is leftuncovered; placing a second tubular covering of expandedpolytetrafluoroethylene, having a length substantially less than thefirst covering length, in contact with the luminal surface of the stent;and forming an encapsulated region by bonding the first tubular coveringto the second tubular covering and leaving a majority of the stentlength unencapsulated.
 6. The process of claim 5, further comprising astep of placing a third tubular covering of expandedpolytetrafluoroethylene, having a length substantially less than thefirst covering length, in contact with the luminal surface of the stent,wherein the second tubular covering and the third tubular covering aredisposed in proximity to a first end and a second end of the stent,respectively, and wherein the step of forming an encapsulated regionfurther comprises bonding the first tubular covering to the thirdtubular covering.
 7. The process of claim 6, wherein the length of thefirst tubular covering is selected and one of the second and the thirdtubular covering is disposed so that one of the first end and the secondend of the stent is left uncovered.